1. Field of the Invention
This invention relates to a printer apparatus capable of varying the ink dot diameter on a recording paper sheet or the mixing density of the ink and a dilution solution for representing the half tone.
2. Description of the Related Art
Among conventional printer apparatus, there is a so-called on-demand type ink jet printer configured for discharging ink liquid droplets via a nozzle responsive to recording signals for recording on a recording medium, such as paper or film sheets. This type of printer is recently coming into widespread use because it lends itself to reduction in size and cost.
On the other hand, computerized document formulation known as desk top publishing has come into use extensively above all in an office. Recently, there is an increasing demand for outputting not only a letter or a figure but also a colored natural image, such as a photograph, along with the letter or the figure. For printing the natural image with high image quality, representation of the half tone is crucial.
In an ink jet printing head used for this on-demand type ink jet printer, a system of employing an electrostrictive oscillator, such as a piezo-element, or a heating element, is generally used as a system for discharging ink liquid droplets. The ink jet printing head is referred to hereinafter as a print head.
The print head exploiting displacement of the electrostrictive vibrator, such as a piezo element, causes ink liquid droplets to be discharged from the nozzle by deforming the piezo element for thereby pressurizing the ink.
As the printer head utilizing displacement of the electrostrictive vibrator, such as the piezo-element, there is so far known such a printer head having a structure of, for example, a vibrating cylinder type, a flat plate vibrating type or a "stemine" type printer head.
Referring to FIG. 1, a flat plate vibrating type printer head, for example, includes a flat plate type electrostrictive vibrator 517, a vibrating plate 518 bonded to the flat plate type electrostrictive vibrator 517 for constituting a so-called bimorph element, a nozzle unit 511 having the bimorph element, an ink chamber 515 defined by the interior of the nozzle unit 511, an ink supply port 514 fed with ink charged into the ink chamber 515, a nozzle 513 and an orifice 516 for discharging the ink as ink liquid droplets 519, and a voltage generator for generating a voltage applied across the bimorph element. The ink droplets 519 are discharged due to volumetric changes in the ink chamber 515 produced by applying the voltage from the voltage generator 512 across the bimorph element.
In the printer head of the on-demand type ink jet printer, the above-described system of employing the heating element, among the systems for emitting the ink liquid droplets, discharges the ink liquid droplets by taking advantage of the phenomenon of vaporization of the ink solution caused by the heating element.
Referring to FIGS. 2A to 2D, the printer head employing the heating element is designed and constructed so that an electric power is applied to a heating element 537 provided in a nozzle 533 provided in a nozzle unit 531 for instantly vaporizing the ink in the nozzle 533 under the resulting heat energy for discharging ink droplets 539 from an orifice 536 under volumetric changes produced under the pressure of bubbles produced due to ink vaporization.
The liquid ink droplets in the printer head employing the heating element are discharged by the following sequence of operations: If the power is applied across the heating element 537, the ink in contact with the heating element 537 is heated to ebullition to form a large number of small-sized bubbles, as shown in FIG. 2A. These small-sized bubbles coalesce to a large-sized bubble 540 and the ink in the nozzle 533 is extruded from the orifice unit 536 under the abrupt pressure rise in the bubble 540, as shown in FIG. 2B. The power supply to the heating element 537 is then discontinued. This diminishes the size of the bubble 540 abruptly such that the pressure in the nozzle 533 is decreased abruptly, as shown in FIG. 2C. The ink extruded from the orifice 536 is separated in this manner from the ink within the nozzle 533 so as to be discharged as the liquid ink droplet 539 as shown in FIG. 2D.
If, in the above-described on-demand type ink jet printer, the half tone is to be represented, the voltage applied to the electrostrictive vibrator, such as the piezo-electric element, or the heating element, or the pulse width, is changed for varying the size of the printing dot formed on the recording sheet for representing the gradation. Alternatively, each pixel is constituted by, for example, a 4.times.4 dot matrix, without varying the picture printing dot diameter, and gradation is represented using a so-called dither method on the matrix basis.
Meanwhile, the above-described conventional on-demand type ink jet printer is frequently constructed by a so-called multi-nozzle structure in which plural nozzles are arranged within a nozzle unit of the printer head.
In the multi-nozzle structure, it occurs frequently that the effective resolution becomes lower than the theoretically feasible resolution, as a result of manufacture tolerances of the respective nozzles, while the picture printing gradation level becomes inaccurate such that printing with uniform printing quality cannot be achieved. In addition, similar problems arise due to deterioration of the nozzle in use with lapse of time. There is also constraint imposed on the individual nozzle structure due to the necessity of reducing the adverse effects caused by deterioration with lapse of time. In addition, it is necessary to make strict control on the characteristics of the ink in use, thus unavoidably raising the cost.